In recent years, to attain both crush safety of a vehicle and weight reduction of a vehicle body, a steel sheet with a higher strength is demanded. However, as the tensile strength of a steel sheet increases, the ductility significantly relating to press formability tends to decrease. Owing to this, a shape that can be formed even with a steel sheet having a low ductility is being studied for example, by simplifying the shape of a press-formed part. A press forming method suitable for a steel sheet having a high strength is being studied.
In case of press forming a steel sheet having a low ductility and a high strength, deep drawing or stamping (bending) is typically employed. For example, a channel part with a simple shape including a vertical wall portion and a top portion continuous to the vertical wall portion but not including a curved portion in the vertical wall portion is manufactured by stamping. Also, a flanged channel part is manufactured by deep drawing.
In stamping, a blank (a flat-sheet-shaped processing material) is arranged on a punch, and the blank is bent with a die, to obtain a product shape. To restrict generation of a wrinkle at a blank portion, which contacts an upper section of the punch, a blank may be pinched and held by the punch and a pad.
In deep drawing, first, a blank holder is arranged at a position corresponding to a flange portion, a blank is arranged on a punch and the blank holder, and a die is arranged above the blank. Then, by lowering the die, the blank is held by the die and the blank holder, and the blank is bent while a load of a proper tensile force is applied to the blank. At this time, the material (the blank) is largely drawn into an area between the punch and the die as the result that the material is held by the die and the blank holder forms a vertical wall portion. Hence, the vertical wall portion is easily formed even when the material has a low ductility.
As a method of adjusting a tensile force, there may be a method of changing a holding force (a cushion pressure) of holding the blank by the die and the blank holder, and a method of arranging a bead at the holding position. If the tensile force applied to the blank is too weak, the material excessively flows to the vertical wall portion, and a wrinkle (a material excess) is likely generated. In contrast, if the tensile force is excessive, the amount of the material flowing to the vertical wall portion is reduced. At forming the vertical wall portion the material is required to be stretched and a crack may be generated if the material has a low ductility.
A press-formed part for a vehicle includes a curved channel part having a curved portion in a vertical wall portion (for example, a lower arm part shown in FIG. 10), and a curved channel part having a flange portion (for example, a center pillar part shown in FIG. 3).
If a curved channel part having a curved portion in a vertical wall portion is manufactured by stamping, when a material is drawn into a vertical-wall-portion formation space of a die and the vertical wall portion is formed, the line length of the material is sufficient at the curved portion, and the material is stretched and deformed in a circumferential direction of the curved portion. This deformation is called “stretch flange deformation.” The stretch flange deformation becomes larger as the material is drawn into the vertical-wall-portion formation space from a position more separated from the curved portion (for example, a portion 42a in FIG. 10 or a portion 22a in FIG. 3). Hence, if the ductility of the material is insufficient in a portion near an outer edge portion of the vertical wall portion, a crack may be generated.
Even when a curved channel part with a flange portion is manufactured by deep drawing, similarly, the flange portion is stretched in the circumferential direction of the above-described curved portion, and hence a crack caused by stretch flange deformation may be generated.
The crack caused by stretch flange deformation is a problem particularly for a material, such as a steel sheet with a high strength, the material which likely has an insufficient ductility. Also, even in a case of a material other than the steel sheet, if the material has a low ductility, a crack caused by stretch flange deformation may be generated. For example, there may be a case in which an aluminum alloy sheet is used for an outer panel of a vehicle for reducing the weight of a vehicle body of the vehicle. In this case, since aluminum alloy tends to have lower press formability than that of a steel sheet, if press forming with stretch flange deformation is executed, a crack may be generated in the outer panel.
To prevent a crack caused by this stretch flange deformation, Patent Literature 1 suggests a method of previously applying a material excess portion (for example, a protruding and depressed shape) at a position of a blank expected to have stretch flange deformation by press forming, and hence preventing the line length of a material from being insufficient in a curved portion during press forming. Also, Patent Literature 2 suggests a method of dispersing stretch flange deformation by an outer edge portion of a vertical wall portion, and hence preventing stretch flange deformation from being locally concentrated.